Colloidal silica is a suspension of very small, spherical particles of amorphous (not crystalline) silica suspended in water. The material is colloidal in that the silica particles do not settle out of the solution. The silica particles generally range in size from about 8 nanometers to a maximum size of about 80 nanometers, beyond which the silica ceases to be colloidal and begins to settle out of solution. Colloidal silica has been used for a variety of purposes, including precision casting, as a lining for molds, as a frictionizing (non-skid) agent, and in a variety of medical and pharmaceutical uses such as toothpaste, dental castings and drug delivery systems.
One relatively recent use which has arisen for colloidal silica is in the polishing of semiconductor materials, such as the silicon wafers used in computer chips, by a process known as Chemical Mechanical Planarization (CMP). CMP involves the polishing of semiconductors and chips using very small abrasive particles of silica alumina, ceria or other materials in a slurry or suspension with a chemically active carrier solution. Colloidal silica has also recently come into use in polishing other items such as hard disk drives, electronic memory devices and raw silicon oxide wafers to accomplish the rough polishing of chips prior to CMP.
The electrical performance of finished semiconductor chips can easily be affected by contaminants to which the semiconductor wafers are exposed during processing. Such contamination can be in the form of discrete particles and water soluble or dispersed organic and inorganic impurities. In particular, the use of silica sols that are contaminated with trace transition metals, alkali and alkaline earth metals, aluminum, and other metals causes difficulties when used in wafer polishing. Sodium, potassium, alkali and alkaline earth metals such as calcium, magnesium, and transition metals such as iron, copper, manganese, nickel, and zinc are particularly troublesome. In general, any transition metal from groups IB, IIB, IIIB, IVB, VB, VIB, and group VIII of the Periodic Table of Elements, if present in high enough concentrations, can cause difficulties in the final products manufactured with silica sols containing these contaminants.
An additional problem of metal contaminants is that many of these substances have much higher diffusivities in both silicon and silicon dioxide than do the more conventional dopants, such as phosphorus and boron. As a result, unpredictable electrical properties can result when silicon wafers are contaminated with these metals. For example, alkali metals such as lithium, sodium, and potassium cause shifts in electrical properties (threshold and flat-band voltages) when incorporated into semiconductor devices.
Currently used colloidal silica CMP slurries generally include a relatively high amount of contaminating sodium, which is very difficult to remove. The contaminating sodium causes defects in the chips in the later manufacturing processes. Accordingly, there is a need for an improved method for producing very low sodium content colloidal silica for use in these applications. There is also a need for an improved method of producing high purity potassium hydroxide which is also an additive used in some CMP slurries.
One method which has been developed in an attempt to produce a low sodium potassium stabilized silica sol is disclosed in U.S. Pat. No. 4,915,870 to Jones. The method disclosed by Jones uses an acid sol process to produce a silica sol having a sodium concentration of less than 150 ppm using commercially available KOH to stabilize the pH of the sol. However, while the method disclosed by Jones is suitable for producing silica sols having a sodium concentration of less than 150 ppm, it would be preferable to produce silica sols having much lower sodium concentrations for use in electronics applications.
In addition to the problem of metal contaminants, current methods for growing colloidal silica particles do not produce larger particles that are uniform in size. Because of the importance in consistency of electrical properties in the manufacture of silicon wafers, it would be useful to be able to produce uniform size colloidal silica particles. Current methods do not produce particles of uniform size because the particles are grown at a rate which causes nucleation of the colloidal silica particles. Methods to produce uniform size colloidal silica particles have been found to be unusable for the production of larger particles because this process requires an impractical growth time due to the limitations on how the particles are grown.
Therefore, it is an object of the present invention to provide a useable system and method for producing uniform colloidal silica particles.
It is another object of the present invention to provide a process for producing colloidal silica particles having a uniform shape and size with a low sodium content.